FIX: Nested NOT Operators Yield Incorrect Value

MS-DOS           | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

If the NOT operator is used more than once in an expression in the Microsoft Macro Assembler (MASM), an incorrect value may be generated. For example:

   mov ax, not(not 0Fh)

   mov ax, FFF1h

RESOLUTION

To work around this, avoid using consecutive NOT operators. Using two NOT operators is equivalent to using none.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and 6.0b. This problem was corrected in MASM for MS-DOS version 6.1.

MORE INFORMATION

Since the NOT operator should perform a bitwise or logical NOT operation, two consecutive NOTs should produce the original number. The sample code below demonstrates the problem.

Sample Code

; Assemble options needed: none

.MODEL small .STACK 4096

.CODE

    .startup
    mov ax, not(not 0fh)
    .exit 0